Lyman-alpha Absorption from Heliosheath Neutrals

Abstract

We assess what information HST observations of stellar Ly-alpha lines can provide on the heliosheath, the region of the heliosphere between the termination shock and heliopause. To search for evidence of heliosheath absorption, we conduct a systematic inspection of stellar Ly-alpha lines reconstructed after correcting for ISM absorption (and heliospheric/astrospheric absorption, if present). Most of the stellar lines are well centered on the stellar radial velocity, as expected, but the three lines of sight with the most downwind orientations relative to the ISM flow (Chi1 Ori, HD 28205, and HD 28568) have significantly blueshifted Ly-alpha lines. Since it is in downwind directions where heliosheath absorption should be strongest, the blueshifts are almost certainly caused by previously undetected heliosheath absorption. We make an initial comparison between the heliosheath absorption and the predictions of a pair of heliospheric models. A model with a complex multi-component treatment of plasma within the heliosphere predicts less absorption than a model with a simple single-fluid treatment, which leads to better agreement with the data. Finally, we find that nonplanetary energetic neutral atom (ENA) fluxes measured by the ASPERA-3 instrument on board Mars Express, which have been interpreted as being from the heliosheath, are probably too high to be consistent with the relative lack of heliosheath absorption seen by HST. This would argue for a local interplanetary source for these ENAs instead of a heliosheath source.

27 pages, 7 figures, AASTEX v5.0, accepted by ApJ

References in corpus (9)

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• Two New Low Galactic D/H Measurements from FUSE
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Cited by in corpus (6)

• The Trouble with the Local Bubble
• Dependence of Heliospheric Lyman-alpha Absorption on the Interstellar Magnetic Field
• Spectral properties of backscattered solar Ly-alpha radiation in the heliosphere: a theoretical search of the heliospheric boundaries effects
• Measuring the Local ISM along the Sight Lines of the Two Voyager Spacecraft with HST/STIS
• New possible way to determine stellar wind terminal velocity from analysis of Lyman-$\alpha$ absorption spectra
• Effects of angular scattering and H+p, H+H collisions on the properties of interstellar atoms in the heliosphere